The lost wax process is a conventionally known method of manufacturing dental prostheses from precious metals or non-precious metals. FIG. 12 is a flowchart showing the process of dental casting according to the lost wax method.
First, a dentist or other individual takes a negative impression model of the mouth and teeth around the object part of a patient (Step S1). Using the negative impression, a dental technician manufactures a dental prosthesis as follows. First, a modeling material, such as gypsum, is poured into the negative impression, and solidifies the material to produce a positive model (Step S2). A desired type of casting model, such as an inlay or crown, for the positive model is created with a heat-subliming material, such as wax (Step S3). A sprue wire for forming a sprue runner is attached to an appropriate part of the casting model with wax (Step S4). After that, the casting model is detached from the positive model, and the free end of the sprue wire is pushed into a crucible former made of rubber (Step S5).
FIG. 13 is a front view of a casting model mounted on a commonly used crucible former. The crucible former 100 has a conical base 101 formed at its center, and a hole 102 for inserting a sprue wire 104 is formed on the top of the conical base 101. The hole 102 is filled with softened wax, and the free end of the sprue wire 104 (to which the casting model 103 is attached) is inserted in the soft wax. When the wax solidifies, the casting model 103 is fixed on the top of the conical base 101 with the sprue wire 104.
A metallic cylindrical ring (not shown) is fitted onto the crucible former 100 so that the casting model 103 is surrounded by the ring, and an investment material, such as gypsum or phosphate, is poured into the metallic ring to conceal the casting model 103 (Step S6). After the solidification of the investment material, the crucible former 100 is removed, the sprue wire 104 is pulled out, and the ring is heated to a high temperature. The heat burns off the wax inside, leaving a cavity corresponding to the sprue wire 104 and the casting model 103. Thus, a mold is obtained (Step S7).
Then, the mold is heated to a preset temperature, and molten metal is poured into a reservoir at the top of the mold, which is a reservoir having a shape corresponding to the conical base of the crucible former. The molten metal flows into the cavity through the sprue runner. This is so-called the “pouring” (Step S8). After the poured metal has cooled down and solidified, the mold is broken to take out the casting inside (Step S9). Then, unnecessary parts such as fringe metals along the sprue runner are removed from the casting, and after-treatments such as sanding the surface of the cast are carried out (Step S10). Thus, the prosthesis is completed.
Conventionally, in the work of Step S7, an electrical furnace, called a “ring furnace”, is used to heat the ring to burn off the wax and to heat the mold to a preset temperature. The pouring work of Step S8 is carried out, for example, using a pressure casting apparatus with an inverting casting chamber (see the Japanese Unexamined Patent Publication No. 2000-176629, for example). In the aforementioned work, the operator should carry the crucible for melting the ring or ingot from one apparatus to another by himself or herself. Therefore, when various forms of prostheses are to be manufactured, it is necessary to spend a considerable amount of time and labor to carry out the casting work as described above. Further, the temperatures and time periods for burning the ring and for melting the metal must be appropriately determined depending on the selection of the investment material and alloy material, because a desired quality of prosthesis cannot be obtained when the settings are inappropriate. Conventionally, however, some pieces of prosthesis result in being defective (e.g. missing a part) as a result of inappropriate settings of the apparatuses, wrong order of work and/or accidental omission of work. This is inevitable when the casting work is manually carried out as described above.
In view of the above problem, the applicant has proposed an apparatus for casting dental prosthesis, disclosed in the Japanese Unexamined Patent Publication No. 2002-85426, which automatically performs the entire work relating to the casting. With this apparatus, the operator needs only to arrange a compound ring with a casting model embedded inside, and a crucible containing alloy ingots as casting material, at predetermined positions, and to set casting conditions as desired. After that, a completed casting is obtained in the ring in a preset period of time.
Though the above apparatus can automatically perform the burning and pouring, it cannot perform the basic work of setting rings and crucibles in the apparatus, which must be manually performed by the operator. Therefore, it is possible that the operator accidentally makes a mistake or omits a work process. Another problem, which is less serious than the aforementioned cases, is that the personal differences or other kinds of unevenness in the manner of working may cause a defect in the prosthesis, decrease the working efficiency or increase the manufacturing costs. Therefore, it has been demanded to provide the above apparatus with additional functionalities for decreasing human errors and other kinds of factors leading to defective products, or for correcting the errors, etc. Examples of the problems are as follows.
(1) The operator should correctly place the ring on a ring-placing platform. A displacement of the ring from the predetermined position often leads to the incorrect gripping of the ring by a gripping device (which is called the “gripper” hereinafter), which may make the ring fall down in the gripping process. To correct the placement of the ring, the apparatus disclosed in the Japanese Unexamined Patent Publication No. 2002-85426 is provided with movable pins used for pushing the ring into the correct position. This, however, requires a mechanism for moving the pins, which accordingly increases the production costs. It is desirable to construct a mechanism for correcting the position of the ring at a lower cost.
(2) As the crucible for melting the alloy ingots, a carbon-made crucible is used for the heating at relatively low temperatures, and a ceramic crucible is used for the heating at the temperature of 1400 C or higher. Ceramic crucibles gradually become brittle when they are repeatedly used. The possibility of cracking or breaking of the crucible rapidly increases when it is used more than approximately twenty times. If such a problem occurs in the course of the pouring process, it is necessary to not only stop the work but also perform other unnecessary work, such as the removal of the scattered casting material.
(3) Rings are burned as they are placed on a lifting stage. In the burning process, residues of burned wax or similar matters are deposited onto the lifting stage. If such residues are not removed, the ring placed on the lifting stage may be tilted by a piece of residue lying underneath, causing it to be on the verge of falling down or other hazardous situation. Accordingly, the operator must frequently take the trouble to clean the lifting stage to remove the residues. Neglecting this job may eventually lead to a discontinuation of the casting work.
(4) The operator may mistakenly set the ring upside down on the ring-placing platform. If this happens, the pouring cannot be correctly performed, which wastes not only time but also the casting material.
(5) For a casting material that has been used before, the appropriate temperature and time for melting the material is already known. For a material whose appropriate temperature and time for melting is unknown, it is often necessary to visually check the molten state of the alloy ingots in the crucible to determine at what temperature and in what time the material is adequately melted. Also, operators having little experience in the work may desire to visually check the molten state of the alloy ingots in the crucible to see whether or not the melting temperature and other parameters have been correctly set. To do such a visual check with conventional apparatuses, the operator is required to lean over the apparatus to look into the crucible, which is not only troublesome but also unsafe.
In view of the above problems, the present invention proposes an apparatus for casting dental prosthesis constructed to reduce the time and labor of the operator and compensate for mistakes and/or unevenness in the work to reduce defective products, improve the working efficiency and avoid the unnecessary increase in production costs.